High-yield visible-to-ultraviolet upconversion by ZnSe quantum dots with dynamic triplet transfer

Abstract

Abstract Triplet–triplet annihilation upconversion sensitized by quantum dot is often limited by the short exciton lifetime, necessitating tethered triplet transmitter. Here, we report a transmitter-free system of ZnSe quantum dot and 2,5-diphenyloxazole achieving 24% upconversion quantum yield, the highest for quantum dot sensitization upconversion. The upconversion quantum yield peaks at 30 mM 2,5-diphenyloxazole. Transient spectroscopy shows the triplet energy transfer efficiency is over 80% regardless of the 2,5-diphenyloxazole concentration, but the triplet–triplet annihilation efficiency dominates the overall upconversion quantum yield and shows a strong dependence on the 2,5-diphenyloxazole concentration that increases first from 10% to 25%, then decreases. The optimal concentration is achieved when the triplet 2,5-diphenyloxazole collision probability is highest, thus ensuring a maximum triplet–triplet annihilation efficiency. This work demonstrates a design principle of binary quantum dot upconversion system and reveals the critical role of modulating emitter concentration for optimal upconversion quantum yield.